(Hope you don't mind someone else weighing in on this conversation...) I'm not a tennis expert but I think that in her best years, when she was on, she hit with such incredible abandon but in the latter stages, she was a lot more, hmm, resilient for lack of a better term. It was interesting to watch Graf steamroll her opponents early in her career because she looked like a genius with a tennis racquet who could do anything with the ball. She even hit topspin backhands as a rally shot and could pass anyone at will with her backhand. In the latter stages of her career, I felt it was so much easier to put pressure on her backhand (although maybe that is also because people just started hitting harder and getting Graf out of position a lot more). I do think she hit a lot more unforced errors in her earlier years.

However, as you have also suggested, her forehand became a much better rallying shot later in her career and she started moving the ball around better. I feel that 1994 may have been one of the biggest turning points in her career. At that time, she lost every tough match she had against ASV and got blown off the court twice by Mary Pierce. Her resolve in tough matches against difficult opponents and her ability to rebound against people who were hitting exceedingly hard off both wings were put into question. But maybe her back problems that nearly ended her career provided some perspective. All of a sudden, in 1995, ASV couldn't beat her anymore after that, even in very tough matches (Wimbledon, French Open). Maybe her experiences playing Pierce helped her develop an ability to hang in there when her opponent seemed to be blowing her away. She had a few matches against Lindsay Davenport where Lindsay racked up 4-5 games in a row but Steffi managed to steady herself and get through. She also lost to Seles 1-6 in the first set of the 1998 year-end championships but somehow managed to get the ball back in a lot more and got through that one as well.

(I sort of think that in 1994, Graf may have felt - even subconsciously - that the only player who could arguably outhit her was Seles. So when she ran into Pierce in the FO, it is possible that her pride - or refusal to accept that anyone could hit harder than her - prevented her from adjusting her game to the situation or slowing down and trying to make a better match out of it. That she remained incredibly competitive against Davenport, Seles, and both Williams sisters very late in her career when her body was already battered may have been a consequence of the realization in late 1994 that she needed to play smarter and hang tougher against these players who could hit her off the court. True enough, she beat Pierce very convincingly early in 1995.)